In vehicles comprising a front and rear vehicle part which are joined together by means of a hinge mechanism, hydraulic steering cylinders are provided between the vehicle parts whereby the vehicle is steered by the vehicle parts being angularly displaced relative to each other by means of the steering cylinders. Such vehicles, which are so-called waist steered vehicles and which are driven on roads, as well as cross-country vehicles, for example forest tractors and other logging machines, are formed with two types of steering mechanisms, that is, a fine steering mechanism for road driving, and a coarse steering mechanism for cross-country driving.
For road driving, small and smooth steering deflections are required in most cases. For the control of the steering cylinders during this fine steering operation the vehicle is provided with a valve designed in a convenient manner and which is controlled by means of a steering wheel. This fine steering operation is also designed in such a manner that the steering deflection will be proportional to the angle of rotation of the steering wheel. The valve used for this purpose may be of the type described in J. L. Rau, Hydrostatic Steering with Power-Beyond Capability, SAE Transactions 740435, Earthmoving Industry Conference, Central Illinois Section, Peoria, Ill., Apr. 23-24, 1974.
For cross-country driving, on the contrary, when it is necessary to make one's way between various obstacles, such as trees, rocks, stumps etc., rapid and large steering deflections are required and it is necessary to facilitate rapid shifting between steering deflections to the left and to the right. The vehicle is then mostly steered by means of a lever which is coupled to a hydraulic valve for controlling the oil flow to the steering cylinders. This type of steering, that is, coarse steering, operates in such a way that, as long as the lever is actuated and the valve is open, oil will flow to the cylinders and the vehicle parts will be continuously displaced angularly relative to each other.
A disadvantage of this construction of the steering of the vehicle is that two separate actuators must be used for the fine and coarse steering operations, that is, a steering wheel and a lever, respectively.